string: Use asm alias instead of symbol redirections for stpcpy and mempcpy

Commit 939da41143 added symbols redirections to handle ISO C
namespace, however some compiler does not support to redeclare the
function prototype.  Moving these defintions to exported header
it not a good practice (it exposes a internal implementation and
it would require to add macros to define it only internally).

Instead this patch replaces the symbol redirections by direct asm
aliases, as done to handle libcall generation done by compiler on
some loop optimizations. The only issue is sparc binutils generates
an extra __mempcpy plt not called anywhere in the code, which indicates
a binutils issue (this is added in the localplt.data for now).

Checked on all affected ABIs.

include/string.h

@@ -179,8 +179,8 @@ extern __typeof (strsep) strsep attribute_hidden;
   && !defined NO_MEMPCPY_STPCPY_REDIRECT
 /* Redirect calls to __builtin_mempcpy and __builtin_stpcpy to call
    __mempcpy and __stpcpy if not inlined.  */
-extern __typeof (mempcpy) mempcpy __asm__ ("__mempcpy");
-extern __typeof (stpcpy) stpcpy __asm__ ("__stpcpy");
+__asm__ ("mempcpy = __mempcpy");
+__asm__ ("stpcpy = __stpcpy");
 #endif
 
 extern void *__memcpy_chk (void *__restrict __dest,

sysdeps/unix/sysv/linux/sparc/sparc32/localplt.data

@@ -17,4 +17,7 @@ libc.so: calloc
 libc.so: free
 libc.so: malloc
 libc.so: realloc
+# Unreferenced PLT created by the symbols aliases used to redirect
+# the compiler generated mempcpy/stpcyp calls done by builtin usage.
+libc.so: __mempcpy
 libm.so: matherr

sysdeps/unix/sysv/linux/sparc/sparc64/localplt.data

@@ -17,4 +17,7 @@ libc.so: calloc
 libc.so: free
 libc.so: malloc
 libc.so: realloc
+# Unreferenced PLT created by the symbols aliases used to redirect
+# the compiler generated mempcpy/stpcyp calls done by builtin usage.
+libc.so: __mempcpy
 libm.so: matherr